Coherent Detection is typically used in communication systems. Spectral coherence between signals is a measure of the relationship between the signals, indicating how well the spectral content of a given signal can be explained by a linear transformation of another signal. For the purposes of this application, coherency is defined as how well a portion of a given signal can be explained based on linear and/or nonlinear transformations of one or more other signals. Coherent detection uses the knowledge of the coherency between 2 or more signals to estimate the information from one or more of the signals.
Oil and Gas exploration uses measurement while drilling (MWD) and logging while drilling (LWD), which involves some combination of directional drilling, formation evaluation, drilling dynamics, and ranging. Directional drilling uses one or more survey instruments aimed at determining position and orientation of the drill bit, bottom hole assembly, well, or other equipment involved in the drilling process. Formation evaluation involves capturing measurements from sensors that respond to various characteristics of the proximate formation, including, but not limited to, natural gamma ray detectors, spectral gamma ray detectors, resistivity sensors, density sensors, and porosity sensors. Drilling dynamics involves capturing measurements from sensors that respond to force, acceleration, velocity, or position of various components of the BHA, including the bit. Drilling dynamics information is typically used to stop or prevent damage to sensors due to vibration and/or provide feedback on parameters that a directional driller is attempting to control, such as weight on bit (WOB) or Torque on Bit. Ranging involves determining relative positioning and orientation between 2 or more locations in the formation, typically between a wellbore being drilled, and one or more nearby wellbores.
Directional drilling may use information on the orientation and position of the bit, such as azimuth, inclination, and toolface, being used to drill a wellbore and a model of the path of the wellbore already drilled to be compared with the expected path, referred to as a well plan. A survey tool is used to provide the base level of information required. Typically, survey data is captured while the survey tool is stationary in order to minimize the noise measured by the sensors of the survey tool(s). Additional information on orientation and position is desired while drilling, but such information is typically less reliable due to the motion of the sensors during the drilling process.
With formation evaluation applications, sensors in the downhole tool are used to collect information about the formation in proximity to the tool to generate a model of the surrounding formation. Sensors typically are rotated within the wellbore in order to measure quantities related to the formation at different angular positions. Typically this rotation is caused by either rotation of the entire drill string by equipment located on the drill rig, or by a mud motor powered by the flow of mud through the drill pipe. In some cases, tools may rotate the sensors through other means. In other cases, the tool may include an array of sensors in order to provide measurements corresponding to different angular positions with respect to the tool. Sensors may include gamma sensors used to measure radiation in the surrounding formation and may be binned to generate the model of the surrounding formation.
Drilling dynamics is the measurement of parameters related to the drilling process that affect the performance of the drilling process in addition to the dynamic environment experienced by the bottom hole assembly (BHA). Measurements that are used for drilling dynamics may include, for example and without limitation, downhole weight on bit, bit torque, bit bounce, BHA whirl, bit whirl, and stick-slip. These measurements typically provide indications of dynamic modes of the BHA, which can be used to adjust various control inputs into the drilling process to make the drilling dynamics in line with some objective of the drilling operator. Such objectives would include, reducing vibration of the tools on the drill string, improving rate of penetration (ROP), avoiding BHA whirl, obtaining a specific weight on bit, and obtaining a specific bit torque.
For each of these applications, noise in the data from the sensors may cause inaccuracies in the generated model. There exists a need to extract more accurate data from the sensors to provide better results in directional drilling, formation evaluation, and drilling dynamics. There exists a need to derive better information regarding the orientation and position of the bit using noisy data to generate a better model of the wellbore. There exists a need to derive better information about the formation in proximity to the tool using noisy data to generate a better model of the formation.